Ubuntu support forums users should be on the lookout for dodgy emails after the website's database of 2 million email addresses was stolen.

Canonical announced the security breach on Friday after being notified that someone was claiming to have a copy of the UbuntuForums.org database. An investigation revealed that an attacker did get access to the website's user records through a vulnerability.

The exploited SQL injection flaw was located in the Forum Runner add-on for vBulletin, commercial web forum software that powers over 100,000 community websites on the Internet and is especially popular with companies. The vulnerability was known, but the Canonical IS team had failed to apply the patch for it in a timely manner.

"The attacker had the ability to inject certain formatted SQL to the Forums database on the Forums database servers," the team said in a blog post. "This gave them the ability to read from any table but we believe they only ever read from the 'user' table."

The user table contained usernames, email addresses and Internet Protocol addresses for 2 million users. It did not contain valid user passwords, but hashed strings that were used for the Ubuntu Single Sign On service. These cannot be used as is to access user accounts.

As a precautionary measure to ensure that no attacker code was left behind, Canonical temporarily took the website down, rebuilt its hosting servers from scratch, installed the most up to date version of vBulletin and reset all system and database passwords.

The company is certain that the attacker was not able to access any code repositories or update mechanisms, did not get write permissions to the Ubuntu Forums database, didn't obtain shell access any of the servers and was not able to mess around with any other Canonical or Ubuntu services.

While there is no immediate danger to Ubuntu Forums accounts, users should be wary of potential spam and phishing emails that might attempt to distribute malware. Attackers typically launch such attacks following large data breaches, since they can take advantage of known relationships between users and the compromised websites.

Ubuntu Forums is not the first large vBulletin-powered website to be compromised. Attackers hacked the MacRumors.com and vBulletin.com forums back in 2013 and Ubuntu Forums itself is at the second major database breach.

Website administrators should always make sure to keep their content management systems up to date, including all third-party add-ons, themes and components they used when building them.

Join the Network World communities on Facebook and LinkedIn to comment on topics that are top of mind.